This application is based upon and claims the benefit of priority from each of the prior Japanese Patent Application No. 2002-36639 filed on Feb. 14, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a DCxe2x80x94DC converter, an electric appliance and a duty-ratio setting circuit.
2. Description of Related Art
A DCxe2x80x94DC converter capable of converting the direct current voltage into a predetermined level has been miniaturized and its efficiency has been highly increased, so that the DCxe2x80x94DC converter has been used for a power supply device of various kinds of electric appliances and its range of use has been increasingly enlarged. In such electric appliances, particularly such as a notebook type of personal computer and portable electric appliances such as a portable phone terminal, the DCxe2x80x94DC converter is an essential device since an IC, an electric circuit, motor and a liquid crystal display device in a main body of the above electric appliances are operated by means of a battery, for example, a primary battery such as an alkaline cell or a secondary battery such as a lithium-ionic cell and a nickel-hydrogen cell as a power supply.
The DCxe2x80x94DC converter, generally, comprises a converter circuit for converting an input voltage into an output voltage having a voltage value different from that of the input voltage in accordance with a switching element turned on and off by a rectangular-wave signal, as well as an output voltage detection circuit for detecting a value of the output voltage to output the detection voltage and a duty-ratio setting circuit for feedback-controlling a duty ratio of the rectangular-wave signal on the basis of the detection voltage so that the output voltage would be controlled at a predetermined value. This structure allows the output voltage of the DCxe2x80x94DC converter to be controlled at a constant value.
Recently, there is a need for the long use of electric appliances, particularly portable electric appliances. It means that there is a requirement of providing a DCxe2x80x94DC converter capable of operating and of supplying a main body of an electric appliance with a predetermined value of output voltage not only in the case that a between-terminal voltage, that is, an input voltage is high since a battery has enough energy, but also in the case that the between-terminal voltage (the input voltage) decreases since the energy is used to be discharged. In other words, a DCxe2x80x94DC converter having a wide operable range of the input voltage is required.
Furthermore, there is also a requirement of a DCxe2x80x94DC converter having a wider setting range of the output voltage so as to be applicable to devices having various specifications.
In a DCxe2x80x94DC converter, the voltage conversion is impossible when the on-duty ratio of the rectangular-wave signal is 100%. As a result, the output voltage in a step-up type of DCxe2x80x94DC converter would decrease to the earth potential, while that of a step-down type would rise up to the input voltage (the voltage of the battery). It is impossible, in practice, that the on-duty ratio exceeds 100%.
In the DCxe2x80x94DC converter, however, the on-duty ratio of the rectangular-wave signal is raised to maintain control in order to set the output voltage at a predetermined value, when the input voltage decreases due to consumption of a battery or the like. Therefore, the on-duty ratio of the rectangular-wave signal finally reaches nearly 100%.
In such case, slight fluctuation of the output voltage due to noise or fluctuation in voltage sometimes causes the on-duty ratio of the rectangular-wave signal, which is set in the duty-ratio setting circuit, to be 100% in calculation. Then, the DCxe2x80x94DC converter cannot output an appropriate voltage, so that the output voltage would decrease (in the case of the step-up type) or rise (in the case of the step-down type). This makes the difference larger between the actual value and the predetermined value of the output voltage and causes the on-duty ratio set in the duty-ratio setting circuit to be increased much more. In the above situation, feedback control is impossible for the DCxe2x80x94DC converter and the output voltage rapidly decreases to the earth voltage or rises to the input voltage. This sometimes makes it impossible to completely carry out a necessary sheltering operation by the time when the power supply of an electric appliance is cut.
In order to prevent the above problem, the following way is taken in some cases. That is, the maximum on-duty ratio possible to be set in the duty-ratio setting circuit is set at a value lower than 100% (80%, for example) in view of noise, fluctuation in voltage and the like. Then, after the duty ratio of the rectangular-wave signal reaches the maximum on-duty ratio to make the feedback control impossible, the output voltage to be generated is determined in accordance with the maximum duty ratio, so that a necessary sheltering operation can be carried out.
In such DCxe2x80x94DC converter, however, the on-duty ratio of the rectangular-wave signal cannot be set in more than the maximum on-duty ratio. Therefore, the operative range of the input voltage is made narrow or the range of the output voltage possible to be outputted is made narrow.
In view of the above, on the basis of the detection voltage of the output voltage detection circuit, a second pulse signal having a fixed on-duty ratio for determining the maximum on-duty ratio is generated separately from generation of a first pulse signal having an on-duty ratio appropriate for feedback control. A DCxe2x80x94DC converter is provided in which a signal having the smaller on-duty ratio is selected to be outputted by a logic process of the first and second pulse signals (see FIG. 1). The above DCxe2x80x94DC converter 100 is a DCxe2x80x94DC converter for raising an input voltage Vin such as a battery, which is inputted to an input terminal Pi, and for outputting an output voltage Vout from an output terminal Po. The DCxe2x80x94DC converter 100 has a converter circuit 110 for converting the input voltage Vin into the output voltage Vout, an output voltage detection circuit 120 for outputting a detection voltage Vd corresponding to the output voltage Vout and a duty-ratio setting circuit 130 for setting an on-duty ratio Don of a rectangular-wave signal PS applied to a switching element M1 on the basis of the detection voltage Vd.
The converter circuit 110 comprises an N-channel of MOS transistor M1, which is a switching element, a coil L1, a capacitor C1 and a diode D1 for preventing a reverse current. The DC input voltage Vin is applied through the coil L1 to a drain of the transistor M1. A source of the transistor M1 is connected to the ground. An anode of the diode D1 is connected to the drain of the transistor M1 while a cathode of the diode D1 is connected to the output terminal Po. The capacitor C1 is connected between the output terminal Po and the ground.
The output voltage Vout outputted from the output terminal Po is made higher than the input voltage Vin in accordance with ON/OFF control of the transistor M1, that is, raised to be outputted. Changing the ratio between ON time Ton and OFF time Toff of the transistor M1 can control the output voltage Vout at a predetermined value.
The output voltage Vout is, concretely, given by an equation of Vout={(Ton+Toff)/Toff} Vin=Vin/Doff. The off-duty ratio Doff is here expressed by an equation of Doff=Toff/(Ton+Toff), while the on-duty ratio Don is expressed by an equation of Don=Ton/(Ton+Toff)=1xe2x88x92Doff. Therefore, Doff+Don=1, and thereby, Vout=Vin/Doff=Vin/(1xe2x88x92Don).
In the output voltage detection circuit 120, the output voltage Vout at the output terminal Po is resistance-divided by resisters R1 and R2 to input the detection voltage Vd into the duty-ratio setting circuit 130.
The duty-ratio setting circuit 130 comprises an error amplifier 131, a triangular-wave oscillator circuit 133, a pulse-width modulator circuit 134, a pulse-signal generator circuit 135 and an AND circuit 137.
An operation of the duty-ratio setting circuit 130 will be described with reference to FIGS. 2 and 3. The error amplifier 131 compares the detection voltage Vd with a reference voltage Vref generated in a reference voltage generator circuit 132 and amplifies a difference voltage between the both voltages Vd and Vref to generate a control voltage Vfb. The triangular-wave oscillator circuit 133 outputs a triangular-wave voltage Vct in a shape of a triangular wave, which changes within a range from the maximum value Vcmax to the minimum value Vcmin (see FIG. 2(A)). When the triangular-wave voltage Vct generated in the triangular-wave oscillator circuit 133 is compared with the control voltage Vfb in the pulse-width modulator circuit 134, a first pulse signal PWO which is pulse-width modulated (PWM) with the control voltage Vfb is generated (see FIG. 2(B)). The triangular-wave oscillator circuit 133 outputs a timing signal TM other than the triangular-wave voltage Vct to the pulse-signal generator circuit 135, the timing signal TM switching a signal level at a timing tmax that the triangular-wave voltage Vct reaches the maximum value Vcmax.
The pulse signal generator circuit 135 generates by means of the timing signal TM a second pulse signal PSO having a comparatively short pulse width and rising at the timing tmax (see FIG. 2(C)). The first pulse signal PWO is inputted to an AND circuit 137 together with a inverted second pulse signal PSOx inverted by means of an inverter 136 from the second pulse signal PSO. Then, the first pulse signal PWO is selected to be outputted as the rectangular-wave signal PS when the control voltage Vfb is sufficiently lower than the maximum value Vcmax of the triangular-wave voltage Vct, as shown in FIG. 2(D) (see FIG. 2(E)). At the same time, the feedback control is carried out to control the output voltage Vfb of the error amplifier 131 so that both of Vout=(R1+R2) Vref/R2 and Vout=Vin/(1xe2x88x92Don) would be simultaneously satisfied, and then, the on-duty ratio Don of the first pulse signal PWO (the rectangular-wave signal PS) outputted from the pulse-width modulator circuit 134 is controlled.
On the other hand, the inverted second pulse signal PSOx is selected to be outputted as the rectangular-wave signal PS when the control voltage Vfb becomes close to the maximum value Vcmax of the triangular-wave voltage Vct and the width of the first pulse signal PWO at a low level is made narrower than that of the inverted second pulse signal PSOx at a low level.
Namely, the on-duty ratio Don having comparatively small value is enough when the input voltage Vin is high due to, for example, no consumption of a battery, and therefore, the first pulse signal PWO is selected in the AND circuit 137 so as to keep the output voltage Vout at a predetermined value. The value of the on-duty ratio Don becomes large, however, when the input voltage Vin decreases due to, for example, consumption of a battery. When the on-duty ratio Don of the inverted second pulse signal PSOx is the smaller value, the inverted second pulse signal PSOx is selected in the AND circuit 137 to output the output voltage Vout given by an equation of Vout=Vin/(1xe2x88x92Don). The maximum value of the on-duty ratio of the rectangular-wave signal PS set in the duty-ratio setting circuit 130 is thus determined, so that the DCxe2x80x94DC converter 100 can be driven up to the maximum on-duty ratio of the inverted second pulse signal PSOx, regardless of noise and voltage fluctuation. In addition, the on-duty ratio Don is prevented from reaching 100%, and therefore, it is also prevented from occurring that the output voltage Vout decreases to the earth voltage or rises to the input voltage Vin due to the on-duty ratio Don reaching 100%. Accordingly, the maximum on-duty ratio, which can be set in the duty-ratio setting circuit 130, can be closer to 100% (90%, for example), so that even lower input voltage Vin can obtain a constant output voltage Vout. Otherwise, a wider range of output voltage Vout can be outputted.
In such DCxe2x80x94DC converter 100, however, since the first pulse signal PWO and the inverted second pulse signal PSOx, which are inputted to the AND circuit 137, are separately generated in the pulse-width modulator circuit 134 and the pulse generator circuit 135, there would be somewhat a difference in respective timing for change due to circuit delay, so that it would be difficult to accord the both timing for change. When the value of the control voltage Vfb is close to the maximum value Vcmax of the triangular-wave voltage Vct, the rectangular-wave signal PS outputted from the AND circuit 137, in some cases, shows because of difference in timing for change between the first pulse signal PWO and the second pulse signal PSO an abnormal waveform in which the first pulse signal PWO overlaps the second pulse signal PSO, as shown in FIGS. 3(A) to 3(E). This may cause instability in operation of the DCxe2x80x94DC converter 100. This is a reason why it is impossible in view of circuit delay to extremely narrow the width of a pulse of the second pulse signal PSO, that is, to extremely increase the on-duty ratios of the inverted second pulse signal PSOx and the second pulse signal PSO. For this reason, it is also impossible in the above mode to completely widen a range of the input voltage and/or the output voltage.
The invention is made in view of the above problems. A purpose of the invention is to provide a DCxe2x80x94DC converter in which no operational instability occurs, a DCxe2x80x94DC converter in which a range of an operable input voltage is wide or a range of an output voltage possible to be outputted is wide, and an electric appliance using such a DCxe2x80x94DC converter. Another purpose of the invention is to provide a duty-ratio setting circuit in which no operational instability occurs in controlling a converter circuit, a duty-ratio setting circuit in which a range of an operable input voltage in a converter-circuit can be widen or a range of an output voltage possible to be outputted can be widen, and an electric appliance using such a duty-ratio setting circuit.
A first aspect of the invention is a DCxe2x80x94DC converter comprising: a converter circuit which includes a switching element and switches the switching element by means of a rectangular-wave signal to convert an input voltage into an output voltage having a value different from a value of the input voltage; an output voltage detection circuit for detecting the value of the output voltage to output a detection voltage; and a duty-ratio setting circuit for generating on the basis of the detection voltage the rectangular-wave signal in which the on-duty ratio is set so that the output voltage would be controlled at a predetermined value and for outputting the rectangular-wave signal to the switching element, wherein the duty-ratio setting circuit comprises: a control-voltage generator circuit for inputting a first reference voltage and the detection voltage to output the control voltage corresponding to a difference therebetween; a first-pulse generator circuit for inputting the control voltage to output a first rectangular pulse signal in which the on-duty ratio changes monotonously with relation to the change of the control voltage; a second-pulse generator circuit for generating a second rectangular pulse signal having a predetermined on-duty ratio; and a selector circuit for selecting one of the first pulse signal and the second pulse signal to output the one as the rectangular-wave signal, in which the first pulse signal is selected and outputted when the control voltage is biased to a first direction compared with the second reference voltage while the second pulse signal is selected and outputted when the control voltage is biased to a second direction compared with the second reference voltage, the first direction being a direction that the on-duty ratio of the first pulse signal becomes smaller as the control voltage is changed, and the second direction being a direction that the on-duty ratio becomes larger to the contrary.
In accordance with the above DCxe2x80x94DC converter, the control voltage and the second reference voltage are compared, and thereby, either the first pulse signal or the second pulse signal would be selected in the selector circuit. Therefore, unlike the related art described above, the rectangular-wave signal in any case shows no abnormal waveform in which the first pulse signal overlaps the second pulse signal when the on-duty ratio becomes large, so that there is no risk that operational instability of the DCxe2x80x94DC converter would occur. Furthermore, it is not necessary to set the on-duty ratio of the second pulse signal in view of difference in timing of change or circuit delay of the both signals.
The first pulse signal is selected when the value of the control voltage is biased to the first direction with reference to the second reference voltage. The first direction is a direction that the on-duty ratio of the first pulse signal becomes smaller upon changing the control voltage. Except for a transitional period such as a beginning time of starting and a switching time from the first pulse signal to the second pulse signal, the control voltage controls the on-duty ratio so that the output voltage would become a predetermined value when the first pulse signal is selected. That is, the feedback control is performed.
On the other hand, the second pulse signal is selected when the value of the control voltage is biased to the second direction with reference to the second reference voltage. The second pulse signal has a predetermined on-duty ratio, and therefore, an output voltage determined on the basis of the predetermined on-duty ratio can be generated in accordance with an equation of Vout=Vin/(1xe2x88x92Don) in a converter circuit. It goes without saying that the predetermined on-duty ratio of the second pulse signal is lower than 100%.
The on-duty ratio denotes in this specification a time rate in which a switching element turns on in a converter circuit. Thus, the on-duty ratio of a pulse signal in which the level is switched between high and low at a timing same as the rectangular-wave signal applied into a switching element is also indicated as the on-duty ratio in the duty-ratio setting circuit, wherein a period for continuing the level on a side corresponding to a time that the switching element turns on is considered to be an ON time. Accordingly, when the high level of a certain pulse signal corresponds to the ON of the switching element, the period for continuing the high level is considered to be the ON time to calculate the on-duty ratio. When the low level of a certain pulse signal corresponds to the ON of the switching element to the contrary, the period for continuing the low level is considered to be the ON time to calculate the on-duty ratio.
The selection in this specification means that a signal to be selected and a signal obtained by the selection are in a relation that they are level-inverted at the same timing. Thus, when the first pulse signal or the second pulse signal inputted to the selector circuit and a rectangular-wave signal outputted are in the relation that they are level-inverted at the same timing for the purpose of a logic process in the selector circuit, the rectangular-wave signal is considered to be the first pulse signal or the second pulse signal, which has been selected, even in a logically inversed relation each other.
Another aspect of the invention is a DCxe2x80x94DC converter comprising: a converter circuit which includes a switching element and switches the switching element by means of a rectangular-wave signal to convert an input voltage into an output voltage having a value different from a value of the input voltage; an output voltage detection circuit for detecting the value of the output voltage to output a detection voltage; and a duty-ratio setting circuit for outputting to the switching element the rectangular-wave signal in which the on-duty ratio is set on the basis of the detection voltage so that the output voltage would be a predetermined value and for carrying out feedback control, wherein the duty-ratio setting circuit generates the rectangular-wave signal having a predetermined on-duty ratio when the on-duty ratio of the rectangular-wave signal set on the basis of the detection voltage reaches substantially 100% and the feedback control becomes impossible.
In a conventional DCxe2x80x94DC converter, the converter circuit can no longer convert voltage when the on-duty ratio of the rectangular-wave signal, which is set on the basis of the detection voltage, reaches substantially 100%, so that the output voltage would start falling or rising, and thereby, the output voltage cannot be maintained at a predetermined value. Then, the feedback control is performed so as to increase the on-duty ratio more, which makes the feedback control impossible, and as a result, the output voltage decreases to the earth potential or rises to the input voltage.
In the DCxe2x80x94DC converter according to the invention, however, the rectangular-wave signal having a predetermined on-duty ratio is generated when the feedback control is impossible. Then, it is possible to generate the output voltage determined in accordance with a predetermined on-duty ratio although the feedback control cannot maintain the output voltage at a predetermined value.
Accordingly, it is possible to give a time for taking measures such as a predetermined sheltering operation against the decrease of the input voltage even when the input voltage decreases due to, for example, consumption of a battery in a notebook-type of personal computer, a portable phone terminal and motor control, which are driven by the DCxe2x80x94DC converter.
In the DCxe2x80x94DC converter according to the invention, the feedback control can maintain the output voltage at a constant value until the on-duty ratio reaches substantially 100%. Thus, it is possible in the DCxe2x80x94DC converter to make a range of the input voltage maximum as well as a range of the output voltage maximum.
The case that the on-duty ratio reaches substantially 100% includes the following case in this specification other than the case that the on-duty ratio reaches 100%. That is, it includes a case that the on-duty ratio of the rectangular-wave signal reaches a large value (99.5%, for example) and the output voltage decreases to the earth potential or increases to the input voltage, as well as the case that a signal having the duty-ratio being 100% is inputted, in accordance with characteristics of other circuit elements such as a switching element, a coil of the converter circuit even when the rectangular-wave signal is inputted into the switching element.
Further aspect of the invention is an electric appliance comprising: the DCxe2x80x94DC converter according to any one the above; a battery for supplying the DCxe2x80x94DC converter with the input voltage; and a main body of the electric appliance driven by using the output voltage of the DCxe2x80x94DC converter.
In such electric appliance, using a battery as a power supply to convert a voltage by means of the DCxe2x80x94DC converter drives a main body of the electric appliance. An operation of the DCxe2x80x94DC converter therefore cannot be unstable when the input voltage to the DCxe2x80x94DC converter decreases due to consumption of a battery to increase the on-duty ratio. Furthermore, it is possible to drive the main body of the electric appliance by obtaining a constant value of the output voltage until the on-duty ratio of a switching element increases too much to continue the feedback control, so that a time period during which the main body of the electric appliance can be used by means of a battery would be extended in some cases.
Moreover, the switching element can be turned on and off in accordance with the rectangular-wave signal having a predetermined on-duty ratio, which is the second pulse signal, even after the feedback control cannot be continued. Then, the output voltage determined in accordance with the predetermined on-duty ratio and the input voltage (the between-terminal voltage of a battery) can be obtained, which enables a necessary sheltering operation such as sheltering of data or system information under a memory to be carried out before the power supply is cut.
The invention is applicable to any electric appliance so long as a battery drives the DCxe2x80x94DC converter in the electric appliance and the output voltage therefrom is used for driving the main body of the electric appliance. Concretely, the applicable electric appliances are, for example, a mobile personal computer of a notebook type or the like, a portable phone terminal, and a mobile sound-recorder, image-recorder and playback equipment for recording and reading out an image, sound or other information in or from a recording medium such as a compact disc and a DVD.
Another aspect of the invention is a duty-ratio setting circuit, which is used for a DCxe2x80x94DC converter comprising a converter circuit which includes a switching element and switches the switching element by means of a rectangular-wave signal to convert an input voltage into an output voltage having a value different from a value of the input voltage and an output voltage detection circuit for detecting the value of the output voltage to output a detection voltage, for setting on the basis of the detection voltage an on-duty ratio of the rectangular-wave signal so that the output voltage would be controlled at a predetermined value, the duty-ratio setting circuit comprising: a control-voltage generator circuit for inputting a first reference voltage and the detection voltage to output the control voltage corresponding to a difference therebetween; a first-pulse generator circuit for inputting the control voltage to output a first rectangular pulse signal in which the on-duty ratio changes monotonously in accordance with the change of the control voltage; a second-pulse generator circuit for generating a second rectangular pulse signal having a predetermined on-duty ratio; and a selector circuit for selecting and outputting one of the first pulse signal and the second pulse signal, in which the first pulse signal is selected and outputted when the control voltage has a value biased to a first direction with reference to the second reference voltage while the second pulse signal is selected and outputted when the control voltage has a value biased to a second direction with reference to the second reference voltage, the first direction being a direction that the on-duty ratio of the first pulse signal becomes smaller as the control voltage is changed, and the second direction being a direction that the on-duty ratio becomes larger to the contrary.
The duty-ratio setting circuit according to the invention is used for a DCxe2x80x94DC converter comprising a converter circuit and an output voltage detection circuit and has a control-voltage generator circuit, a first-pulse generator circuit and a second-pulse generator circuit as well as a selector circuit to select in accordance with the result of comparison between the control voltage and the second reference voltage either the first pulse signal having an on-duty ratio changing in accordance with the control voltage or the second pulse signal having a predetermined on-duty ratio. Thus, the rectangular-wave signal in any case shows no abnormal waveform that the first pulse signal overlaps the second pulse signal, when the on-duty ratio becomes large, unlike the case of the duty-ratio setting circuit 130 in the DCxe2x80x94DC converter 100 according to the related art described above. There is therefore no risk that operational instability of the DCxe2x80x94DC converter 100 would occur, and further, it is not necessary to set the on-duty ratio of the second pulse signal in view of difference in timing of change or circuit delay of the both signals.
Moreover, in accordance with the duty-ratio setting circuit, the rectangular-wave signal (the first pulse signal) having an on-duty ratio corresponding to the control voltage can be outputted when the control voltage is biased to the first direction with reference to the second reference voltage. Thus, controlling ON and OFF of a switching element of the converter circuit can perform the feedback control so that the output voltage is maintained at a constant value. On the other hand, the rectangular-wave signal (the second pulse signal) having a predetermined on-duty ratio can be outputted when the control voltage is shifted to the second direction, that is, when the control voltage changes to a direction that the on-duty ratio increases, and thereby, exceeds the second reference voltage. Since the on-duty ratio of the rectangular-wave signal applied to a switching element never reaches 100%, the output voltage cannot decrease to the earth potential and cannot increase to the input voltage even in the case that the control voltage applied to the duty-ratio setting circuit greatly changes in the second direction because of decrease of the input voltage of the DCxe2x80x94DC converter, which is caused by consumption of a power supply battery, for example.
Yet another aspect of the invention is a duty-ratio setting circuit used for a DCxe2x80x94DC converter comprising a converter circuit which includes a switching element and switches the switching element by means of a rectangular-wave signal to convert an input voltage into an output voltage having a value different from a value of the input voltage and an output voltage detection circuit for detecting the value of the output voltage to output a detection voltage, the duty-ratio setting circuit outputting to the switching element the rectangular-wave signal in which the on-duty ratio is set on the basis of the detection voltage so that the output voltage would become a predetermined value and carrying out feedback control, and the duty-ratio setting circuit generating the rectangular-wave signal having a predetermined on-duty ratio, when the on-duty ratio of the rectangular-wave signal set on the basis of the detection voltage reaches substantially 100% and the feedback control becomes impossible.
In a conventional DCxe2x80x94DC converter, the converter circuit can no longer convert voltage when the on-duty ratio of the rectangular-wave, which is set on the basis of the detection voltage, reaches substantially 100%, so that the output voltage would start falling or rising, and thereby, the output voltage cannot be maintained at a predetermined value. Then, the feedback control is performed so as to increase the on-duty ratio more, which makes the feedback control impossible, and as a result, the output voltage falls to the earth potential or rises to the input voltage.
In the DCxe2x80x94DC converter using a duty-ratio setting circuit according to the invention, however, the rectangular-wave signal having a predetermined on-duty ratio is generated in the duty-ratio setting circuit when the feedback control is impossible. Then, it is possible to generate an output voltage determined in accordance with a predetermined on-duty ratio although the feedback control cannot maintain the output voltage at a predetermined value.
Accordingly, it is possible to give a time for taking measures such as a predetermined sheltering operation including sheltering of data or system information under a memory against the decrease of the input voltage even when the input voltage decreases due to consumption of a battery, for example, in a notebook-type of personal computer, a portable phone terminal and an electric appliance requiring motor control, which are driven by the DCxe2x80x94DC converter using the duty-ratio setting circuit according to the invention.
Moreover, in the DCxe2x80x94DC converter using the duty-ratio setting circuit according to the invention, the feedback control can maintain the output voltage at a constant value until the on-duty ratio reaches substantially 100%. Thus, it is possible in the DCxe2x80x94DC converter using the duty-ratio setting circuit to make a range of the input voltage wide as well as a range of the output voltage wide.
Another aspect of the invention is an electric appliance comprising: a DCxe2x80x94DC converter comprising a duty-ratio setting circuit according to any one of the above, a converter circuit and an output voltage detection circuit; a battery for supplying the DCxe2x80x94DC converter with the input voltage; and a main body of the electric appliance driven by using the output voltage of the DCxe2x80x94DC converter.
In such electric appliance, using a battery as a power supply to convert a voltage by means of the DCxe2x80x94DC converter using the duty-ratio setting circuit drives a main body of the electric appliance. An operation of the DCxe2x80x94DC converter therefore cannot be unstable, when the input voltage to the DCxe2x80x94DC converter decreases due to consumption of a battery to increase the on-duty ratio. Furthermore, it is possible to drive the main body of the electric appliance by obtaining a constant value of output voltage until the on-duty ratio of a switching element increases too much to continue the feedback control, so that a time period during which the main body of the electric appliance can be used by means of a battery would be extended in some cases.
Moreover, the switching element can be turned on and off in accordance with the rectangular-wave signal having a predetermined on-duty ratio, which is the second pulse signal, even after the feedback control cannot be continued. Then, the output voltage determined in accordance with the predetermined on-duty ratio and the input voltage (the between-terminal voltage of a battery) can be obtained, which enables a necessary sheltering operation such as sheltering of data or system information under a memory to be carried out before the power supply is cut.
The invention is applicable to any electric appliance so long as a battery drives the DCxe2x80x94DC converter using the duty-ratio setting circuit in the electric appliance and the output voltage therefrom is used for driving the main body of the electric appliance. The applicable electric appliances are, for example, a mobile personal computer of a notebook type or the like, a portable phone terminal, and a mobile sound-recorder, image-recorder and playback equipment for recording and reading out an image, sound or other information in or from a recording medium such as a compact disc and a DVD.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.